Showing papers on "Depolarization ratio published in 1996"

Determination of first hyperpolarizability of nonlinear optical chromophores by second harmonic scattering using an external reference

Abstract: First hyperpolarizabilities (β) of two tricyanovinylthiophene nonlinear optical chromophores were determined using second harmonic, hyper‐Rayleigh, scattering The use of an external standard in the determinations is compared with the previous use of an internal standard The first hyperpolarizability of the well‐known chromophore 4 dimethylamino 4′ nitrostilbene (DANS) was also determined using the external standard method and its value is compared to those in the literature A new method of analyzing the hyper‐Rayleigh scattering signal by recording a histogram of the scattered energy is reported This method is shown to give more reliable results in a shorter period of time than the usual static gate method The histogram function provides additional information in the form of the histogram width which is shown to be an indication of the mean‐square concentration fluctuations of the chromophores in solution The effects of molecular interactions on the concentration dependence of the hyper‐Rayleigh scattering signal is discussed It is shown that depending on the concentration range, the β value of para‐nitroaniline, used as the external standard, can agree with two different values reported in the literature Results on the depolarization ratio of the hyper‐Rayleigh scattered light from one of the tricyanovinylthiophene chromophores are presented At low concentration the depolarization ratio agrees with the theoretically predicted value based on C2v symmetry However, at higher concentration the measured depolarization ratio increases indicating molecular interactions

Resonance Raman Examination of the Electronic Excited States of Glycylglycine and Other Dipeptides: Observation of a Carboxylate→Amide Charge Transfer Transition

Abstract: We have examined the UV resonance Raman and the VUV absorption spectra of aqueous glycylglycine and other dipeptides. We observe strong resonance Raman enhancement of the amide I, II, and III bands and the amide CαH bending mode in a manner similar to that we observed previously with excitation within the π→π* transition of N-methylacetamide (Chen, X. G.; Asher, S. A.; Schweitzer-Stenner, R.; Mirkin, N. G.; Krimm, S. J. Am. Chem. Soc. 1995, 117, 2884). However, in addition, we observe strong resonance Raman enhancement of the ca. 1400 cm-1 symmetric COO- stretching vibration, whose 206.5 nm Raman cross section is increased 20-fold compared to that of the carboxylate in sodium acetate, for example. Addition of a methylene spacer between the amide and carboxylate groups causes the resonance Raman enhancement of this symmetric COO- stretch to disappear. The UV resonance Raman excitation profiles, the Raman depolarization ratio dispersion, and the VUV absorption spectra of glycylglycine and other dipeptides d...

Complete Determination of Intermolecular Spectral Densities of Liquids Using Position-Sensitive Kerr Lens Spectroscopy

Abstract: Femtosecond position-sensitive Kerr lens spectroscopy is demonstrated to facilitate the direct and complete measurement of the third-order material response function, R(3)(t). The real parts of tensor element response, (t) and (t), for liquid CS2 have been measured and are quantitatively compared to the optical Kerr effect response, (t) + (t). The intermolecular Raman spectral densities are also obtained for each tensor element through Fourier transform. The relationship among these spectral densities is discussed with regard to the nature of the underlying intermolecular motions in liquid CS2. Specifically, we find that the intermolecular depolarization ratio is frequency independent with a value of 0.7 ± 0.1 in the 0−150 cm-1 region.

Homogeneous ice nucleation observed in single levitated micro droplets

Abstract: Homogeneous ice nucleation rates in single isolated micro droplets of water are determined under various atmospheric conditions inside an electrodynamic Paul-trap. The droplet size is measured by angle-re- solved detection of the scattered light from a Helium-Neon laser. The freezing process is detected by a sudden change of the depolarization ratio which is an indication for the formation of non spherical particles. The experimentally determined homogeneous ice nucleation rates JLS are JLS = (2.2 0.4)·10 8 cm -3 s -1 at T = (-37.2 0.15)°C and JLS = (5.6 1.0) ·10 7 cm -3 s -1 at T = (-37.7 0.15)°C. These values are much more precise as determined by measurements inside a cloud expansion chamber. The method of single particle obser- vation eliminates complications arising from these nucleation rate meas- urements. The results underline the theory that one single ice germ of certain size is sufficient to induce the freezing of the whole droplet. The depolarization ratio of the scattered intensity in the parallel and perpendicular polarization plane I s / Ip from the frozen particles exhibits a broad distribution with a mean value of I s / Ip= 0.3.

Depolarization Ratio Measurements in the Atmospheric Boundary Layer by Lidar in Tokyo

Abstract: The atmospheric boundary layer over the central area of Tokyo was studied by a Mie-scattering polarization lidar. Negative correlation of the depolarization ratio with relative humidity was observed in the lower atmosphere throughout the year. Contributions of hygroscopic or water-soluble and deliquescent aerosol particles, especially sea-salt particles to the depolarization ratio are discussed in the light of the chemical compositions of the surface aerosol in Tokyo.

Raman Lidar Observations: Simultaneous Measurements of Water Vapor, Temperature and Aerosol Vertical Profiles, Part II

Abstract: A Raman lidar was developed to observe vertical profiles of water vapor, temperature and aerosols. The lidar system can detect signal from 5 detectors simultaneously. Two vibrational Raman backscattering echoes from H 2 O and N 2 (or O 2 ) excited by Nd:YAG 3rd harmonics wavelength (355 nm) are used to observe the profiles of water vapor mixing ratio and atmospheric temperature (or density). Nd:YAG fundamental and 2nd harmonics wavelengths (1064 and 532 nm) are used to observe backscattering coefficient profiles of aerosols and clouds at these wavelengths. The scattering at 532 nm is observed in parallel and perpendicular components corresponding to the linear polarization plane of the transmitted linearly-polarized laser pulse to observe depolarization ratio profiles. Observed water vapor and temperature distributions coincide with the simultaneously observed radiosonde sounding data excellently. The Raman lidar makes it possible to observe the vertical profiles of water vapor, temperature, aerosols, and clouds simultaneously.

A Possible Origin of Linear Depolarization Observed at Vertical Incidence in Rain

Abstract: Recent observations by two different nadir-pointing airborne radars with some polarization capabilities have defected surprisingly large linear depolarization ratios at times in convective tropical rain. This depolarization can he explained if the rain is considered to be a mixture of a group of apparent spheres and another group of drops that are distorted in the horizontal plane perpendicular to the direction of propagation of the incident wave. If confirmed in future observations, this suggests that at times the larger raindrops are oscillating, in part, because of collisions with smaller drops. Since many of the interpretations of the polarization measurements in rain by ground-based radars presume that the raindrop shapes correspond to those of the well-known “equilibrium” drops, the present observations may require adjustments to some radar polarization algorithms for estimating rainfall rate, for example, if the shape perturbations observed at nadir also apply to measurements along other a...

Multiple scattering from Chebyshev particles: Monte Carlo simulations for backscattering in lidar geometry

Abstract: Lidar measurements are often interpreted on the basis of two fundamental assumptions: absence of multiple scattering and sphericity of the particles that make up the diffusing medium. There are situations in which neither holds true. We focus our interest on multiply-scattered returns from homogeneous layers of monodisperse, randomly oriented, axisymmetric nonspherical particles. T(2) Chebyshev particles have been chosen and their single-scattering properties have been reviewed. A Monte Carlo procedure has been employed to calculate the backscattered signal for several fields of view. Comparisons with the case of scattering from equivalent (equal-volume) spheres have been carried out (narrow polydispersions have been used to smooth the phase functions' oscillations). Our numerical effort highlights a considerable variability in the intensity of the multiply-scattered signal, which is a consequence of the strong dependence of the backscattering cross section on deformation of the particles. Even more striking effects have been noted for depolarization; peculiar behavior was observed at moderate optical depths when particles characterized by a large backscattering depolarization ratio were employed in our simulations. The sensitivity of depolarization to even small departures from sphericity, in spite of random orientation of the particles, has been confirmed. The results obtained with the Monte Carlo codes have been successfully checked with an analytical formula for double scattering.

Dicarbocyanine dyes in methanol solution probed by depolarized Rayleigh and hyper‐Rayleigh light scattering

Abstract: The hyper‐Rayleigh scattering (HRS) intensity of two symmetric carbocyanine dyes (1122 DEDC and 1144 DEDC, full names given in the text) in methanol is measured as a function of dye concentration. These dye molecules at equilibrium show a negligible permanent dipole moment. The low concentration data showing that the HRS intensity is proportional to the dye concentration are used to determine the first hyperpolarizability for each of these dyes. However, above a concentration ρb=0.1×10−3 M, the HRS intensity shows an anomalous concentration dependence. Above ρb, the HRS intensity shows a saturation behavior and it even decreases with increasing concentration at high dye concentration. The depolarization ratio of the HRS intensity is also measured as a function of dye concentration. At lowest concentration, the depolarization ratio is 0.18. As the dye concentration increases, the depolarization ratio also rapidly increases but the increase quickly saturates as the concentration exceeds ρb. The concentratio...

Polarized Micro Raman Spectroscopic Studies of Nucleic Acid: Local Raman Tensors in Inosine-5′-monophosphoric Acid and the Orientation of Ekypoxanthine Residue in Poly(rI). Poly(rC) Fiber ‡

Abstract: The polarized Raman spectra of a single crystal of the barium salt of inosine monophosphoric acid hexahydratc (Ba-IMP.6H2O) have been observed with 488.0 nm excitation. For each Raman band, the relative intensities of aa, bb, cc, ab and bc tensor components have been determined. The tensor quotients from the crystal were augmented with measured depolarization ratios in solution. From these experimental data, the shape and orientation of the localized Raman scattering tensor were deduced for each of the normal modes of the hypoxanthine residue, phosphate moiety and ribose portion. The hypoxanthine residue gives a strong Raman band around 1553 cm−1, which shows rather large depolarization ratio, p = 0.32, in aqueous solution, and shows a great scattering anisotropy in the single crystal of IMP. The shape and orientation of the Raman tensor associated to this 1553 cm−1 vibration have been determined: one of its principal axes (y-axis) is directed along the long axis (N1-N7) of the hypoxanthine resid...

Raman light scattering from supracritical binary fluid mixtures: CH4/CF4

Abstract: A supracritical fluid mixture of CH4/CF4 (1:9 mole ratio) has been studied at 323 K at densities between 3.0 and 19.1 mol ‐l (m/l) by inelastic (Raman) light scattering originating from the ν1 totally symmetric stretching mode of methane. Furthermore, a Raman depolarization ratio study of the integrated intensities of the ν1 mode was also carried out as a function of density. A model is proposed to aid in the understanding of the intensity‐density behavior in terms of both allowed and interaction‐induced (ii) contributions to the overall observed signal. The model makes use of one, two, and three body light scattering via both the dipole polarizability, the dipole–quadrupole polarizability, and also takes into account various partner combinations in the multibody light scattering. Whereas, the model generally predicts correct Iνh behavior, (here I is the Raman intensity and the subscripts refer to the vertical polarization direction of the laser (ν) and the direction of the analyzer either ν or h (horizon...

Depolarization Ratio of Air Molecules in the Free Troposphere-Lidar Measurement in Spring 1994-

Abstract: The depolarization ratio of atmospheric molecules was estimated to be 0.021 on the basis of regression analysis of total depolarization ratio and scattering ratio of atmospheric aerosols measured by a lidar in spring, 1994.